Broad band filters in respirator canisters or in gas-filtering elements offer the user comprehensive protection against a great variety of harmful gases. Such a comprehensive protective action of a gas filter imposes high requirements on the chemical performance of the activated carbons used. To enable such activated carbons to remove the greatest possible variety of chemically different reactive harmful gases, these activated carbons must be chemically impregnated.
A large number of inorganic and organic compounds, for example, basic copper and zinc carbonates, copper and zinc sulfates, sodium dichromate, sodium molybdate, sodium vanadate, ammonium chloride, ammonium carbonate, silver nitrate as well as TEDA (triethylenediamine) or tartaric acid and salts thereof are known for impregnating activated carbons for use in gas filters. Chromate, which is often classified as carcinogenic by experts, has been regularly eliminated in impregnations of activated carbons for use in gas canisters. The necessary chemical action is ensured, instead, among other things, by molybdates and vanadates.
Copper-containing activated carbons are known as well. An essential drawback of the copper-containing activated carbons is the possibility of formation of cyanogen upon admission of HCN after storage of the filter. Cyanogen possesses toxic properties comparable to those of HCN, but it cannot be perceived by humans. Consequently, the user could not recognize the breakthrough of his or her breathing filter in an environment contaminated with HCN and could even run the risk of becoming poisoned with cyanogen without noticing it. To ascertain that the user of a breathing filter can perceive the breakthrough of his or her filter, which may possibly have already been stored for a rather long time, in an HCN-contaminated environment, the activated carbon contained in the filter must not form cyanogen at any time.
Cyanogen is formed from HCN in the presence of copper compounds in a basic environment, more precisely by Cu(II) salts, which react while undergoing reduction to Cu(I). The formation of cyanogen by Cu2+ is as follows:Cu2++2 HCN−>CuII(CN)2+2H+CuII(CN)2−>CuI(CN)2+½ (CN)2.
The presence of a sufficient quantity of suitable oxidants, for example, chromates, molybdates or vanadates, on the copper-containing activated carbon can prevent the release of cyanogen.
Should the ratio of copper to the oxidant decrease, e.g., due to aging effects, the risk of release of cyanogen will increase (Table 1). It has now been found that the formation of cyanogen can be avoided by eliminating the use of Cu(II) compounds.
Besides the prior-art activated carbons, which are free from chromates but do, however, contain molybdate, there also are impregnated activated carbons in the area of broad band filters, which activated carbons do entirely without oxo anions of groups V and VI. Impregnations that contain neither oxo anions of groups V and VI nor copper salts are known as well. The activated carbons are impregnated with small quantities of very expensive silver salts in both cases and sometimes also contain larger quantities of zinc salts. However, the silver salts used for the impregnation predominantly react on the activated carbon by forming elemental silver, which has formally no oxidizing action. Since neither copper salts nor oxidants are present on such an activated carbon, such activated carbons have a markedly poorer performance with respect to SO2 and H2S. Even though this property can be partially compensated by increasing the percentage of water on the activated carbon, the separation capacity for organic non-polar harmful gases is lowered by this measure. Furthermore, the storage of filters with activated carbon with high water content should be considered with markedly more criticism. Drying out of the activated carbon leads to major loss of performance, e.g., in case of the adsorption of SO2 or NH3.